Earthquakes are caused by the resultant relative slippage of the earth crust, generally along or near major tectonic plate boundaries. In certain parts of the world, continuous differential movement occurs between one section of the earth's crust and an adjacent one, causing an accumulation of strain at the boundary. When the stresses caused by this strain accumulation exceed the strength of the earth's materials, a slip occurs between two portions of the earth's crust and tremendous amounts of energy are released. This energy propagates outward from the focus or origin of the earthquake in the form of body and surface elastic stress waves.
The energy released during an earthquake event is transmitted through the earth's crust in the form of body and surface seismic waves. The body waves are composed of P-(compression) waves and S-(shear) waves, with the P-wave traveling significantly faster than the S-wave. The surface waves of most interest are the Rayleigh wave and the Love wave. The Love wave travels faster than the Rayleigh wave. The total energy transported is represented almost entirely by the Rayleigh, the S- and the P-waves, with the Rayleigh wave carrying the largest amount of energy, the S-wave an intermediate amount and the P-wave the least. The velocity of the P-wave is almost double that of the S-wave, and the velocity of the S-wave is only slightly greater than the Rayleigh wave.
At some distance from an earthquake disturbance a particle at the earth's surface first experiences a displacement in the form of an oscillation at the arrival of the P-wave followed by a relatively quiet period leading up to another oscillation at the arrival of the S- and Rayleigh waves. These events are referred to as the minor tremor and the major tremor at the time of arrival of the Rayleigh wave. The body and surface waves are monitored during an earthquake to gauge the earthquake's intensity.
Earth ground motions experienced during an earthquake are actually quite complex due to the variation in the earth's crust, from strong stiff bedrock to soft weak soils. Considerable energy can be transmitted through the bedrock, and it appears that in many cases the main forces acting on soil elements in the field during earthquakes are those resulting from the upward migration of shear motions from the underlying rock formations. Although the actual wave pattern may be very complex, the resulting repeated and reversing shearing deformations, imposed on the soil by the S-wave components are the principal cause of liquefaction in saturated fine sand or silty sand deposits. When these soil deposits are subjected to repeated shear strain reversals, the volume of the soil decreases with each cycle, i.e. the soil contracts, and due to the lack of drainage of these saturated soils, the soil pore water pressure rises. As the soil pore water pressure rises, the grain to grain contact pressure becomes smaller, until eventually the grain to grain contact pressure drops to zero and the soil loses all of its shear strength and acts like a fluid. This phenomenon is known as liquefaction and can occur in loose saturated fine sands and silty sands as a result of earthquakes, blasting or other shocks.
The factors that effect the occurrence of liquefaction are soil type, grain size distribution, compactness of the soil, soil permeability, magnitude and number of the strain reversals. Fine cohesionless soils, fine sand or fine cohesionless soils containing moderate amounts of silt are most susceptible to liquefaction. Uniformly graded soils are more susceptible to liquefaction than well graded soils, and fine sands tend to liquefy more easily than coarse sands or gravelly soils. Moderate amounts of silt appear to increase the liquefaction susceptibility of fine sands; however, fine sands with large amounts of silt are less susceptible, although liquefaction is still possible. Recent evidence indicates that sands containing moderate amounts of clay may also be liquefiable.
In very coarse sands or gravel ground water can flow freely enough that pore water pressures never become dangerously high to give rise to liquefaction. Fine sands and silty sands however have moderate to low permeability, which prevents the dissipation of induced pore water pressures and results in liquefaction of the soil. If the soil pore water pressures generated during an earthquake event can be relieved then the soil will not liquefy and hence will remain stable.
The temporary loss of shear strength resulting during liquefaction can have a catastrophic effect on earthworks or structures supported on such soils. Major landslides, settling or tilting of buildings and bridges and instability of dams or tailings ponds have all been observed in recent years and efforts have been directed to prevent or reduce such damage.
Conventional soil stabilization methods to minimize or prevent liquefaction consist of one of five general methods:
1) remove liquefaction prone soil material and replace with sound material,
2) provide structural support to underlying firm soil strata, e.g. piling,
3) densify the soil to render it less susceptible to liquefaction,
4) strengthen the liquefaction prone soils,
5) provide drainage to prevent build up of soil pore water pressures, e.g. stone or gravel columns or relief wells.
The above methods have proven successful in minimizing liquefaction related damage; however, they are expensive, difficult to implement in existing structures and some of the methods are severely limited in their effectiveness in fine grain soils.
Under certain static soil conditions, soil stabilization may be achieved by electro-osmosis. Electro-osmosis involves the application of a constant, low d-c current between electrodes inserted in the saturated soil, that gives rise to pore fluid movement from the source electrodes towards the sink electrodes and thus modifies the soil pore water pressures. Electro-osmosis has been used in applications such as 1) improving stability of excavations, 2) decreasing pile driving resistance, 3) increasing pile strength, 4) stabilization of soils by consolidation or grouting, 5) dewatering of sludges, 6) groundwater lowering and barrier systems, 7) increasing petroleum production and 8) removing contaminants from soils. Electro-osmosis uses a low level d-c electrical potential difference applied across the saturated soil mass by electrodes placed in an open or closed flow arrangement. The d-c potential difference sets up a low level constant d-c current flowing from the source electrodes to the sink electrodes. In most soils the soil particles have a negative charge. For those negatively charged soils, the source electrode is the anode electrode, the sink electrode is the cathode electrode, and ground water migrates from the anode electrode toward the cathode electrode. In other soils, such as calcareous soils (e.g. limestone), the soil particles carry a positive charge. In those positively charged soils, the source electrode is the cathode electrode, the sink electrode is the anode electrode, and ground water migrates from the cathode electrode toward the anode electrode.
An "open" flow arrangement at the electrodes allows an ingress or egress of the pore fluid. Due to the electrically induced transport of pore water fluid, the soil pore water pressures are modified to enable excavations to be stabilized or pile driving resistance to be lowered. Electro-osmosis is not used extensively due to the high cost of maintaining the d-c potential over long periods of time and the drying out and chemical reactions that occur if the system is activated for long periods of time. For short term stabilization by pore water pressure reduction, electro-osmosis is very effective in fine grained soils, such as fine sands, silty sands and silts.
Because of the damage caused by earthquakes, various methods of forecasting earthquakes have been attempted in order to reduce damage and loss of life. Forecasting an impending earthquake requires an identification and monitoring of physical parameters which are often referred to as precursors of seismic activity. Monitoring the early arrival ground motion due to seismic waves using accelerometers has enabled real time forecasting of an impending major earthquake tremor, but such forecasting only provides warnings by a matter of generally seconds although sometimes up to a minute. Such forecasting does not provide timely warning of an impending earthquake to allow evacuation or other normal emergency preparation, and considerable effort has been directed to forecasting an impending earthquake from monitoring other activities, such as changes in animal behavior, build-up of strain in the rocks of the earth's crust, changes in P-wave velocities, uplift and tilt of the earth, changes in ground water levels in wells, increases in the emission of radon gas, and changes in the earth's resistivity, magnetic and electromagnetic fields or currents. These other more timely methods of forecasting impending earthquakes are currently not sufficiently precise to predict the onset of an earthquake or a major earthquake tremor.
Monitoring ground motion and activating safety devices prior to the arrival of a major earthquake tremor can in some cases reduce damage. Such a forecasting system can be used to close gas valves or cutoff electricity to the effected area. Such systems may include a tuned pendulum system, that upon the onset of certain ground motion magnitude and frequency, the pendulum motion sets off an alarm, activates a switch or closes a gas valve prior to the arrival of the major tremor of the earthquake. Alternatively, a heavy sliding or rotating mass can be used to activate a similar switch, contact or value, by sizing the mass that upon experiencing certain ground motions the mass slides or rotates and activates a switch, contact or closes a valve prior to the arrival of the major destructive earthquake tremor.